Sorry my friend, Its about the most taxing.
its common sense you run outside you sweat from trying to stay cool and the fatigue of your weight while runing
swimming youre more weightless on top of that the water keeps you cool so you dont expend more calories
running on a hot day all you want is someone to throw some water on your head to cool off and recover your energy
but its all relative because say when you run you push yourself as hard as YOU CAN GO same with swimming you can only go as fast as YOU CAN GO both reaching similar fatigues in similar times if you pushed yourself exactly the same
its much more comfortable swimming though